1. Field
Embodiments of the present invention relate to integrated circuits and, in particular, to integrated circuit fabrication processes.
2. Discussion of Related Art
It is common to use a process known as atomic layer deposition (ALD) to deposit films on semiconductor wafers to fabricate transistors, for example. In ALD, the semiconductor (e.g., silicon, germanium) wafer is placed in a reactor. A precursor material is pulsed into the reactor. The precursor material subsequently adsorbs and reacts on the wafer surface. The precursor material may be any one of hundreds of possible materials, depending on the reaction product (i.e., metal oxide film, metal nitride film, etc.) desired. The reactor is then purged with an inert gas to remove the precursor material. A second reactant material is pulsed into the reactor. The second reactant material reacts with the precursor material on the wafer surface. Selection of the second reactant material depends on the reaction product desired and on which precursor material was selected. The reactor is purged again.
This process of precursor pulsing, reactor purging, reactant pulsing, and reactor purging is called a “cycle.” In ALD, the thickness of the deposited film is controlled by the number of cycles.
In current ALD processes, where metal oxide films are to be deposited on silicon wafers, the films are deposited using preset conditions for the reactor and materials used that remain constant throughout the deposition process. There are drawbacks to using preset conditions, however.
For example, using one discrete set of preset conditions may produce a film that has good insulation against current leakage but also causes silicon oxide or metal silicate at the interface between the silicon wafer surface and the metal oxide film to be formed. Interfacial silicon oxide or metal silicate can be problematic because these materials have relatively low permittivity, which reduces the effective dielectric constant of a transistor's gate stack.
A reduced effective dielectric constant results in reduced capacitance, which reduces transistor drive current for a given dielectric film thickness. When the transistor drive current is reduced the speed performance of the device degrades. Using another discrete set of preset conditions may produce a film having very little to essentially no interfacial silicon oxide or metal silicate but produces a film with poor insulation against current leakage.